Membrane electrode assembly, method for producing the same, and solid state polymer fuel cell

ABSTRACT

A membrane-electrode assembly (MEA)  1  has a solid polymer electrolyte membrane  2 . The membrane  2  has ion-conductive domains  3  and non-ion-conductive domains  4  and an electrode catalyst  5 . The electrode catalyst  5  is present selectively on surface sites of the solid polymer electrolyte membrane  2  which corresponds to the ion-conductive domains  3  rather than surface sites of the membrane  2  which corresponds to the non-ion-conductive domains  4 . A spray liquid containing the electrode catalyst and a solvent is applied onto a surface of the membrane  2  by electrostatic spray deposition to selectively adhere the electrode catalyst  5  on the surface sites of the membrane  2  which corresponds to the ion-conductive domains  3 . The membrane  2  is preferably subjected to a hydrophilization treatment before being sprayed with the spray liquid.

TECHNICAL FIELD

This invention relates to a membrane-electrode assembly having anelectrode catalyst adhered to selected sites of the surface of a solidpolymer electrolyte membrane and a process of producing the same. Thepresent invention also relates to a solid polymer electrolyte fuel cellhaving the membrane-electrode assembly.

BACKGROUND ART

Solid polymer fuel cells have recently been researched and developedextensively. As part of the research and development, solid polymerelectrolytes having high proton conductivity have been studied from theaspects of conductivity, chemical and thermal stability, and economicalefficiency. The origin of solid polymer fuel cells can be traced to theGemini 5's on-board polymer fuel cells, which were supplanted by alkalifuel cells on account of the low performance of the solid polymerelectrolyte used therein. Then, Du Pont Company developed Nafion (aregistered trademark for a perfluoroalkylsulfonic acid polymer). Thehigh proton conductivity and chemical and thermal stability possessed byNafion have again boosted development of solid polymer fuel cells. Theinventors of the present invention previously proposed a solid polymerion conductor in which polymer molecules having an ionically dissociablegroup are oriented in an electric field in an attempt to provide a solidpolymer electrolyte with high proton conductivity and thermal andchemical stability (see Patent Document 1).

Central to the solid polymer fuel cell technology is a thin film device,which is a laminate of a solid polymer electrolyte membrane andelectrodes, called a membrane-electrode assembly (hereinafterabbreviated as MEA). An MEA has contributed to size and weight reductionof fuel cells and driven practical application of fuel cells forvehicles and domestic use. As illustrated in FIG. 10, in a currentlyavailable MEA 1′, a solid polymer electrolyte membrane 2′ generally hasa phase-separated structure composed of hydrophilic (ion-conductive)domains 3′ and hydrophobic (non-ion-conductive) domains 4′. An electrodecatalyst 5′, which is adhered to the solid polymer electrolyte membrane2′, is applied to the entire surface of the solid polymer electrolytemembrane 2′. Not all the electrode catalyst 5′ participates in electrodereaction, nevertheless. Only the part of the electrode catalyst that isin contact with the hydrophilic domains 3′ serving for ionic conductioncan participate in electrode reaction. The part of the electrodecatalyst applied to the hydrophobic domains 4′ is not givenopportunities to take part in the reaction. In other words, thestate-of-the-art MEAs have a large quantity of an electrode catalystthat does not participate in electrode reaction.

Patent Document 1: JP-A-2003-234015

Accordingly, an object of the present invention is to provide an MEAfree from the above-mentioned problem associated with the related artand a process of producing the MEA.

DISCLOSURE OF THE INVENTION

The present invention accomplishes the above object by providing an MEAwhich has a solid polymer electrolyte membrane. The membrane hasion-conductive domains and non-ion-conductive domains and an electrodecatalyst. The electrode catalyst is present selectively on surface sitesof the solid polymer electrolyte membrane which corresponds to theion-conductive domains rather than surface sites of the electrolytemembrane which corresponds to the non-ion-conductive domains.

The invention also provides a preferred process for producing the MEA.The process comprises applying a spray liquid containing the electrodecatalyst and a solvent onto a surface of the solid polymer electrolytemembrane by electrostatic spray deposition.

The invention also provides a process for producing an MEA comprisingthe steps of:

discretely applying an ion-conductive liquid to a surface of a solidpolymer electrolyte membrane which is substantially free from adissociated proton, and then

applying a spray liquid containing an electrode catalyst and a solventonto the surface of the solid polymer electrolyte membrane byelectrostatic spray deposition to adhere the electrode catalystselectively to the part of the solid polymer electrolyte membrane wherethe ion-conductive liquid has been applied.

The invention also provides a solid polymer electrolyte fuel cell havingthe MEA and a separator which is disposed on each surface of the MEA.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-section of an MEA according to the presentinvention.

FIG. 2 is a scanning electron micrograph image taken of a surface of theelectrolyte membrane used in Example 1.

FIG. 3 schematically illustrates apparatus for carrying out an ESDmethod.

FIG. 4( a) and FIG. 4( b) are a plan and a longitudinal cross-section ofa second embodiment of the MEA according to the present invention.

FIG. 5 is a scanning electron micrograph image taken of a surface of theelectrolyte membrane of the MEA prepared in Example 1.

FIG. 6 is a scanning electron micrograph image taken of a surface of theelectrolyte membrane of the MEA prepared in Comparative Example 1.

FIG. 7 is a photograph presenting the appearance and the results of apeel test of the samples obtained in Example 4 and Comparative Example2.

FIG. 8 is a scanning electron micrograph image of a cross-section of thesample obtained in Example 6.

FIG. 9 is a chart showing the results of elemental analysis on platinumon the cross-section of the sample obtained in Example 6.

FIG. 10 schematically illustrates a conventional membrane-electrodeassembly.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described based on its preferredembodiments with reference to the accompanying drawings. In FIG. 1 isshown a schematic cross-section of an MEA according to the presentinvention. The MEA 1 shown in FIG. 1 has a solid polymer electrolytemembrane 2 and an electrode catalyst 5 applied to both surfaces of themembrane 2.

The electrolyte membrane 2 has ion conductivity, such as protonconductivity. The electrolyte membrane 2 has a number of ion-conductivedomains 3 with ion conductivity (e.g., proton conductivity) and a numberof non-ion-conductive domains 4 with no ion conductivity. Theion-conductive domains 3 and the non-ion-conductive domains 4 arephase-separated from each other. The ion-conductive domains 3 are sitesthat participate in electrode reaction, whereas the non-ion-conductivedomains 4 are inert sites that do not substantially participate inelectrode reaction. FIG. 1 schematically depicts the ion-conductivedomains 3 and the non-ion-conductive domains 4 as if they extended thewhole width of the electrolyte membrane 2 for the sake of facilitatingunderstanding the invention. In an actual electrolyte membrane someion-conductive domains can be disconnected halfway in the widthdirection of the membrane. The same applies to the non-ion-conductivedomains.

FIG. 2 presents a scanning electron micrograph image taken of a surfaceof the electrolyte membrane used in Example 1 given later. As isapparent from FIG. 2, a number of nearly circular, shallow depressionsare observed on the surface of the electrolyte membrane. Thesedepressions correspond to the ion-conductive domains, and the portionsurrounding the ion-conductive domains is the non-ion-conductivedomains.

Back to FIG. 1, an electrode catalyst 5 is applied to each of surfaces 6of the electrolyte membrane 2. What should be noted here is that theelectrode catalyst 5 is applied not on the entire area of the surfaces 6but selectively on the sites corresponding to the ion-conductive domains3 rather than the sites corresponding to the non-ion-conductive domains4. In the MEA 1 of the present embodiment, the electrode catalyst 5 isapplied selectively on the surface sites of the ion-conductive domains 3that participate in electrode reaction. In brief, the electrode catalyst5 is applied selectively to where it is essentially needed. As a result,the amount of the catalyst to be used can be reduced without affectingthe performance of the MEA. Considering that the cost of MEA productionaccounts for about 75% of the production cost of the current solidpolymer electrolyte fuel cells, reducing the amount of the electrodecatalyst 5 by selective application makes tremendous contributions toreduction of the cost of fuel cells.

FIG. 1 displays a state in which the electrode catalyst 5 is appliedonly to the sites of the surfaces corresponding to the ion-conductivedomains 3 with no electrode catalyst 5 given to the surface sitescorresponding to the non-ion-conductive domains 4. Depending on thephase separation conditions between the ion-conductive domains 3 and thenon-ion-conductive domains 4 in the electrode membrane 2 and thedeposition conditions (hereinafter described) of the electrode catalyst5, the electrode catalyst may also be applied to the surface sitescorresponding to the non-ion-conductive domains 4. Even in such cases,the electrode catalyst 5 is applied in a larger amount to the surfacesites corresponding to the ion-conductive domains 3 than to the surfacesites corresponding to the non-ion-conductive domains 4.

Materials making up the electrolyte membrane 2 are typically exemplifiedby perfluorocarbonsulfonic acid resins, which are proton-conductivepolymers. Perfluorocarbonsulfonic acid resins are preferred for theirexcellent chemical and thermal stability. Examples of this kind ofresins include Nafion (a registered trademark of E.I. de Pont de Nemours& Co, U.S.A.), Aciplex (a registered trademark of Asahi ChemicalIndustry C.o., Ltd.), and Flemion (a registered trademark of Asahi GlassCo., Ltd.). Other useful polymers include sulfonated polyether ketoneresins, sulfonated polyether sulfone resins, sulfonated polyphenylenesulfide resins, sulfonated polyimide resins, sulfonated polyamideresins, sulfonated epoxy resins, and sulfonated polyolefin resins.

The electric field-oriented, solid polymer ion conductor the presentinventors proposed in JP-A-2003-234015 supra is also useful as amaterial constituting the electrolyte membrane 2. The electricfield-oriented solid polymer ion conductor is obtained by orienting apolymer having an ionically dissociable group in an electric field. Sucha polymer includes one prepared by polymerizing a monomer having aprotonic acid group, such as a carboxyl group, a sulfonic acid group ora phosphoric acid group. Examples of the monomer are acrylic acid,methacrylic acid, vinylsulfonic acid, styrenesulfonic acid, and maleicacid. The electric field-oriented solid polymer ion conductor maycontain a polymer having no ionic group. Examples of the polymer havingno ionic group include fluoroalkyl polymers such as polyvinylidenefluoride, polytetrafluoroethylene, vinylidene fluoride-trifluoroethylenecopolymers, vinylidene fluoride-hexafluoropropylene copolymers, andpolytetrafluoroethylene-ethylene copolymers; alkyl polymers such aspolyethylene, polypropylene, chlorinated polyethylene, and polyethyleneoxide; and polymers having a substituted or unsubstituted arylene groupin the main chain thereof, such as polycarbonate, polyester, polyestercarbonate, and polybenzimidazole. The electric field-oriented solidpolymer ion conductor may contain, in addition to the polymer having anionically dissociable group and the polymer with no ionic group, acompatibilizer which is compatible with both of these polymers. Usefulcompatibilizers include known surface active agents, polyvinyl acetal,polyvinyl butyral, and polyvinylpyrrolidone. Also included arepolyethylene glycol methacrylate, polyethylene glycol dimethacrylate,and polymers or oligomers obtained by copolymerizing the monomerproviding these homopolymers and a copolymerizable monomer having, ifdesired, a hydroxyl group, an ester group, an amido group, a carbamoylgroup, a sulfamoyl group, etc.

The electric field-oriented solid polymer ion conductor is obtained bydissolving or dispersing the polymer having an ionically dissociablegroup, the polymer having no ionic group that is used if desired, andthe compatibilizer compatible with both the polymers that is used ifdesired in a solvent and subjecting the solution or dispersion to thestep of electric field orientation by means for electric fieldorientation while the solvent is being removed.

What is called “a pore-filling polymer”, i.e., a non-ion-conductiveporous polymer the pores of which are filled with an ion-conductivepolymer can also be used as a material of the electrolyte membrane 2.The non-ion-conductive porous polymer includes porouspolytetrafluoroethylene and polyimide nonwoven fabric made mainly ofcrystalline polyimide fiber disclosed in JP-A-2004-185973. Theion-conductive polymer filling the pores of the non-ion-conductiveporous polymer includes acrylic acid-sodium vinylsulfonate copolymers,perfluorocarbonsulfonic acid resins, polystyrenesulfonic acid resins,sulfonated polyether ether ketone resins, sulfonated polyphenylenesulfide resins, polyimide resins having a sulfonic acid group, andphosphoric acid-doped polybenzimidazole resins.

Filling the pores of a non-ion-conductive porous polymer with anion-conductive polymer can be achieved by, for example, penetrating asolution of a monomer providing the ion-conductive polymer into thepores of the non-ion-conductive porous polymer and causing the monomerto polymerize to form the ion-conductive polymer in the pores. Inanother method, a solution of an ion-conductive polymer is infiltratedinto a non-ion-conductive polymer, followed by solvent removal to fillthe pores with the ion-conductive polymer.

Whatever kind of the electrolyte membrane 2 may be selected, thethickness of the membrane is not critical in the invention and may bedecided as appropriate for the balance between strength and resistanceof the membrane. Usually, a thickness of about 10 to 200 μm, preferablyabout 30 to 100 μm will be enough. When the electrolyte membrane 2 isformed by solution casting, the membrane thickness can be controlled bythe solution concentration or the coating thickness on a substrate.Where the electrolyte membrane 2 is formed of a molten polymer, thethickness can be controlled by stretching a film of predeterminedthickness formed by melt-pressing, melt-extrusion, etc. to apredetermined stretch ratio.

Any electrode catalysts that have hitherto been used in the art can beused as the electrode catalyst 5 with no particular restriction.Examples include platinum, gold, silver, palladium, iridium, rhodium,ruthenium, iron, cobalt, nickel, chromium, tungsten, manganese,vanadium, and alloys thereof. Among them, platinum is usedpredominantly. The metal particles as a catalyst usually have a particlesize of 10 to 300 Angstroms. The catalyst can be adsorbed on a carriersuch as carbon particles, which results in reduction of the amount ofthe catalyst to be used, offering an economical advantage. In view ofthe cost performance, a preferred amount of the catalyst to be used isabout 0.001 to 10 mg/cm².

While not shown in FIG. 1, the MEA 1 of the present embodiment has, inaddition to the electrode catalyst 5, a gas diffusion layer provided onboth surfaces of the electrolyte membrane 2 to form an oxygen electrodeand a fuel electrode. The gas diffusion layer functions as a supportingcurrent collector having current collecting capabilities. The gasdiffusion layer also serves to supply sufficient gas to the electrodecatalyst 5. Useful gas diffusion layers include carbon paper and carboncloth. More specifically, the gas diffusion layer can be formed ofcarbon cloth woven of a 1:1 mixture of polytetrafluoroethylene-coatedcarbon fiber and non-coated carbon fiber. The gas diffusion layer isprevented from being completely covered with water and thereby exhibitssatisfactory gas permeability, since the carbon fiber is water repellentdue to the polytetrafluoroethylene coating.

A separator is disposed on both sides of the MEA 1 of the presentembodiment to provide a solid polymer electrolyte fuel cell. Theseparator has ribs extending in one direction at a prescribed spacing onits side facing the gas diffusion layer. Every adjacent ribs formtherebetween a groove having a rectangular section. The grooves serve asflow channels for feeding and discharging a fuel gas or an oxidizing gas(e.g., air). The fuel gas and the oxidizing gas are fed from fuel gasfeeding means and oxidizing gas feeding means, both not shown in theFIG. 1. The separators on both sides of the MEA 1 are disposed withtheir grooves crossing at right angles. The above-describedconfiguration constitutes the minimum cell unit. Several tens to severalhundreds of these cell units are stacked to make up a fuel cell stack.

A preferred process of producing the MEA 1 of the present embodimentwill then be described. One of the characteristics of the processresides in use of electrostatic spray deposition (hereinafterabbreviated as ESD) as a technique for applying the electrode catalyst 5to the electrode membrane 2. FIG. 3 schematically illustrates an exampleof an ESD apparatus 10. A plate 12 made of an electron conductivematerial such as metal is placed on a flat mount 11. An electrolytemembrane 2 is put on the plate 12. Separately, a nozzle 13 ejecting aspray liquid is arranged above the electrolyte membrane 2 so as to facethe electrolyte membrane 2. With an electric field applied between themetal plate 12 and the nozzle 13, a spray liquid is sprayed onto theelectrolyte membrane 2 whereby an electrode catalyst is appliedselectively on the surface sites corresponding to the ion-conductivedomains 3 of the electrolyte membrane 2.

The spray liquid sprayed from the nozzle 13 is a dispersion containingthe catalyst. Examples of a dispersing medium include aqueous media,such as a mixture of water and alcohol, and water. The spray liquidpreferably contains an ion-conductive polymer to improve adhesion of thecatalyst to the electrolyte membrane 2. While the polymer to be used isnot particularly limited, to sue the same polymer as used to constitutethe electrolyte membrane 2 is preferred to ensure adhesion of thecatalyst.

The nozzle 13 preferably has a cross-sectional area of its openings ofabout 0.01 to 1 mm². The spray liquid is stored in a tank (not shown),delivered to the nozzle by feeding means such as a pump, and sprayedfrom the nozzle. The amount of the spray liquid to be sprayed is decidedappropriately according to the catalyst concentration of the liquid andthe amount of the catalyst to be applied to the electrolyte membrane 2.Usually, it is preferably about 1 to 1000 μl/min. For the same reasons,the time of spraying is preferably about 5 to 500 seconds.

The distance between the openings of the nozzle 13 and the electrolytemembrane 2 is preferably 5 to 200 mm, still preferably 10 to 100 mm, toassure uniformity of catalyst application and to prevent splash of thecatalyst on an area other than the electrolyte membrane 2. The voltageapplied between the nozzle 13 and the metal plate 12 is a direct currentvoltage. To make a proper spray and to spray the catalyst homogeneously,the voltage to be applied preferably ranges from 5 to 30 kV, stillpreferably 8 to 20 kV. While FIG. 3 shows voltage application with thenozzle 13 connected to a plus terminal and the metal plate 12 to a minusterminal, the polarities of the direct current voltage applied may bereversed.

It is preferred that the electrolyte membrane 2 be subjected to ahydrophilization treatment (a treatment for activating ion conductivity)prior to spraying the spray liquid. The hydrophilization treatment makesit possible to apply the electrode catalyst 5 with high selectivity tothe ion-conductive domains. The hydrophilization treatment can beaccomplished by immersing the electrolyte membrane 2 in ion exchangewater or an aqueous solution of a mineral acid (e.g., sulfuric acid,hydrochloric acid or nitric acid) having a concentration of about 0.01to 5 N for a prescribed time (e.g., about 1 to 100 minutes).

ESD can be carried out in the air at room temperature. After completionof spraying the spray liquid onto the electrolyte membrane 2, the mediumof the spray liquid is allowed to evaporate in the same environment togive the electrolyte membrane 2. Thereafter, a gas diffusion layer isdisposed on each side of the electrolyte membrane 2 to produce the MEA1.

According to the above-described process, use of ESD makes it possibleto deposit the catalyst selectively on the ion-conductive domains of theelectrolyte membrane 2. ESD is generally known as a technique in whichan electric field is applied between a nozzle ejecting a metalion-containing solution and a metallic substrate to form a metal oxidethin film on the heated substrate. It is also known as a technique forforming various thin films including a carbon thin film. However, ESDhas not been made use of as a technique for applying a functionalmaterial (a noble metal-based electrode catalyst in the process of theinvention) selectively to conductive parts rather than to non-conductiveparts of a substrate (i.e., an electrolyte membrane).

A second embodiment of the present invention is then described withreference to FIGS. 4( a) and 4(b). The description with respect to thefirst embodiment applies appropriately to the particulars of the secondembodiment that are not described hereunder. The members of FIG. 4 thatare the same as in FIG. 1 are given the same numerical references as inFIG. 1.

As illustrated in FIGS. 4( a) and 4(b), the MEA 1 of the presentembodiment is of non-bipolar stacking type. The MEA 1 has a number ofelectrode catalyst adhesion regions 7 a discretely provided on onesurface of a solid polymer electrolyte membrane 2 and the same number ofelectrode catalyst adhesion regions 7 b discretely provided on the othersurface of the electrolyte member 2 at positions opposite to theelectrode catalyst adhesion regions 7 a across the electrolyte membrane2. Depending on the method and the conditions of adhering the electrodecatalyst, a small amount of the electrode catalyst can adhere to regionsother than the adhesion regions 7 a and 7 b. Even in such cases, a muchlarger amount of the electrode catalyst is adhered in the adhesionregions 7 a and 7 b than in the other regions.

Each of pairs of the adhesion regions 7 a and 7 b facing each otheracross the electrolyte membrane 2 constitutes a single cell 8. The MEA 1has a number of such single cells 8 connected in series via aninterconnector 9. The adhesion regions 7 a and 7 b making up one pairfacing across the electrolyte membrane 2 are of the same shape. Theshape, while rectangular in the present embodiment, is not particularlylimited. The area of the individual adhesion regions 7 a and 7 b is notparticularly limited, either. Considering that non-bipolar stacking issuited to micro fuel cells, the area of the individual adhesion regions7 a and 7 b is preferably 0.1 to 500 cm², still preferably 0.5 to 100cm².

Since bipolar separators are not used in a non-bipolar stackconfiguration adopted in the present embodiment, the MEA 1 of thepresent embodiment achieves size and weight reduction and increasedenergy density of fuel cells more easily than the MEA having a bipolarplate stacking configuration.

FIG. 1 corresponds to an enlarged view of the individual adhesionregions 7 a and 7 b formed on both surfaces of the electrolyte membrane2. That is, each adhesion region 7 a or 7 b has both the surface sitescorresponding to the ion-conductive domains 3 (see FIG. 1) and thesurface sites corresponding to the non-ion-conductive domains 4 (seeFIG. 1). Each adhesion region 7 a or 7 b has the electrode catalystadhered selectively on the surface sites corresponding to theion-conductive domains rather than on the surface sites corresponding tonon-ion-conductive domains. Accordingly, when viewed macroscopically,the electrolyte membrane 2 of the MEA 1 of the present embodiment haselectrode catalyst adhesion regions 7 a and 7 b and regions with noelectrode catalyst adhered. When viewed microscopically, each of theadhesion regions 7 a and 7 b also has sites with the electrode catalystadhered (the surface sites corresponding to the ion-conductive domains)and sites free of the electrode catalyst (the surface sitescorresponding to the non-ion-conductive domains). Thus, the MEA 1 of thepresent embodiment has the electrode catalyst adhered with extremelyhigh selectivity. Therefore, the present embodiment makes it feasible toreduce the amount of the expensive catalyst including a noble metalwithout being accompanied by impairment of the performance of the MEA 1.

In a conventional MEA of non-bipolar stack configuration, occurrence ofcrossover between adjacent single cells has been prevented by disposinga non-ion-conductive material, such as engineering plastics, between thesingle cells, i.e., between adhesion regions 7 a (or 7 b) neighboring onthe sample plane. However, such an MEA has a complicated structure andrequires labor and cost for the production. According to the presentembodiment, in contrast, a large number of single cells can befabricated on a single electrolyte membrane 2, which offers theadvantage that the MEA structure is not complicated. Furthermore, use ofthe process of production hereinafter described facilitates selectiveformation of the adhesion regions 7 a and 7 b. To prevent crossover fromoccurring and to achieve battery downsizing, the distance betweenneighboring adhesion regions 7 a (or 7 b) on the same plane ispreferably about 1 to 20 mm, still preferably about 2 to 10 mm.Understandably, an insulator such as an engineering plastic and ceramicsmay be applied between adjacent single cells on one or both surfaces ofthe electrolyte membrane 2, which will be effective to ensure insulationagainst electron and ion conduction.

A preferred process of producing the MEA 1 of the embodiment shown inFIGS. 4( a) and 4(b) will be described below. First of all, a solidpolymer electrolyte membrane is prepared. A solid polymer electrolytemembrane is generally available in a dry state which has not been givena treatment for rendering ion-conductive. An electrolyte membrane inthat state is substantially free from dissociated protons. Theexpression “substantially free from dissociated protons” as used hereindoes not mean that there are no dissociated protons but that existenceof a trace amount of dissociated protons resulting from adsorption ofmoisture in the air is acceptable. An ion-conductive liquid isdiscontinuously applied to one surface of the electrolyte membrane ofthat state. For example, a microsyringe is used to apply anion-conductive liquid discretely on one surface of the electrolytemembrane. When the liquid is applied in a rectangular pattern, arectangular adhesion region will be formed by ESD as hereinafterdescribed. The electrolyte membrane may be dried in vacuo to ensuresubstantial freedom from dissociated protons.

The ion-conductive liquid includes water, a dilute aqueous acidsolution, and a lower alcohol. The dilute aqueous acid solution includesan about 0.01 to 5N aqueous solution of a mineral acid such as sulfuricacid, hydrochloric acid or nitric acid. The lower alcohol includes C1 toC4 alcohols, particularly methanol and ethanol.

The present inventor's study has revealed that addition of a smallamount of the ion-conductive liquid to the electrolyte membrane issufficient. An electrode catalyst adhesion region can be formed on thesurface of the electrolyte membrane by applying the ion-conductiveliquid in an amount as small as about 1 to 300 μl, preferably about 20to 100 μl, per square centimeter, which varies depending on the kind ofthe electrolyte membrane.

The electrolyte membrane having the ion-conductive liquid appliedthereto is then subjected to the aforementioned ESD process. In carryingout ESD, the electrolyte membrane is placed with its surface having beengiven the ion-conductive liquid facing with the nozzle. ESD is carriedout under the conditions previously stated.

An electrode catalyst is deposited selectively by the ESD process on theregions where the ion-conductive liquid has been applied, whereby manydiscrete adhesion regions are formed on one surface of the electrolytemembrane, and the sites where the electrode catalyst is applied aregiven ion conductivity. The reason accounting for the selectiveapplication of the electrode catalyst has not been made clear as yet.According to the inventors' assumption, an electric field applied to theion-conductive liquid in ESD causes the ion-conductive liquid toinstantaneously penetrate into the electrolyte membrane to impart ionconductivity to the impregnated portion, which may account for theselection adhesion of the electrode catalyst.

Microscopic observation of the region where the electrode catalyst hasbeen deposited, i.e., the adhesion region proves that the electrodecatalyst has been applied selectively onto the surface sitescorresponding to the ion-conductive domains in the region rather thanonto the surface sites corresponding to the non-ion-conductive domains,which is similar to the case with the embodiment shown in FIG. 1.

The time from application of the ion-conductive liquid to theelectrolyte membrane to ESD is not critical. What is noteworthy is thatselective application of an electrode catalyst can be accomplished toimpart ion conductivity to the electrode catalyst adhesion region eventhough the above-mentioned time is extremely as short as severalminutes. In order to make an electrolyte membrane ion-conductive, it hasbeen said to be necessary that the electrolyte membrane should beimmersed in, e.g., an aqueous acid solution at room temperature or whileboiling for several hours to about 24 hours. In contrast, theabove-described process enables an electrolyte membrane to be madeion-conductive selectively in an extremely short time.

By the above-described operation, a number of adhesion regions arediscretely formed on one surface of the electrolyte membrane.Subsequently, the electrolyte membrane is reversed, and ESD is carriedout on the other surface. It is not necessary to apply theion-conductive liquid to the other surface of the electrolyte membranebecause the electrolyte membrane has already been renderedion-conductive by the first ESD.

The electrode catalyst is thus applied to the other surface of theelectrolyte membrane by the second ESD. The regions to which theelectrode catalyst is applied are where the electrolyte membrane hasbeen made ion-conductive. In other words, the electrode catalyst isapplied in the second ESD to the regions opposite to the regions wherethe electrode catalyst has been applied in the first ESD across theelectrolyte membrane. Thus, the process of the present embodimentenables application of the electrode catalyst, i.e., formation ofadhesion regions, on the same positions as the regions opposite acrossthe electrolyte membrane without requiring a special operation forpositioning.

EXAMPLES

The present invention will now be illustrated in greater detail withreference to Examples, but it should be understood that the invention isnot construed as being limited thereto. Unless otherwise noted, all thepercents are given by weight.

Example 1 (1) Preparation of Catalyst Dispersion

Platinum-on-carbon (20% Pt) was finely ground with water in an agatemortar and mixed with a mixed alcohol solution(methanol:2-propanol:water=1:1:1 by weight). A 5% solution of aperfluorocarbonsulfonic acid resin (Nafion, a registered trademark of DuPont) was added thereto to prepare a catalyst dispersion. The resultingcatalyst dispersion had a platinum-on-carbon to perfluorocarbonsulfonicacid resin weight ratio of 1:1. The solids concentration was adjusted to1.7%.

(2) Preparation of Electrolyte Membrane

Polyacrylic acid, polyvinyl butyral, and a fluororesin (Cefral Soft,available from Central Glass Co., Ltd.) at a weight ratio of 3:1:6 weredissolved in 15 parts of dimethylformamide under heating. The resultingsolution was applied to a glass substrate. An external electric field of8000 V/cm was applied to the coating layer with means for electric fieldorientation until the solution dried to the touch to form a thin film.The glass substrate with a polymer thin film formed thereon was dried byheating at 150° C. for 1 hour in a vacuum dryer. After cooling, thepolymer thin film was stripped off the glass substrate to obtain anelectrolyte membrane having a thickness of 40 μm. The electrolytemembrane was immersed in a 1N sulfuric acid aqueous solution for 5minutes and washed thoroughly with ion exchanged water to complete ahydrophilization treatment (ion conductivity activation treatment).

(3) ESD

ESD was performed using the apparatus 10 shown in FIG. 3. A metal plate12 which was a gold electrode was placed on the mount 11. Theelectrolyte membrane 2 prepared in (2) above was put on the metal plate12. The distance between the nozzle 13 and the electrolyte membrane 2was set at 40 mm. With an electric field of 12 kV applied between thenozzle 13 and the metal plate 12, the spray liquid was sprayed from thenozzle 13 onto the electrolyte membrane 2 at room temperature in theair. The nozzle 13 was connected to a positive pole, and the metal plate12 a negative pole. The spray liquid was fed at a rate of 33.33 μl/minand sprayed for 15 seconds. After ESD, the sprayed liquid was dried atroom temperature in the air. The electrolyte membrane was reversed, andESD was conducted on the other surface in the same manner. The sprayedliquid was dried at room temperature in the air, whereby an electrolytemembrane having an electrode catalyst deposited on each surface thereofwas obtained. The amount of the applied electrode catalyst at eachsurface was 0.03 mg/cm². Carbon cloth was overlaid on each surface ofthe electrolyte membrane as a gas diffusion layer to give an MEA.

Comparative Example 1

The catalyst dispersion of Example 1 was directly dropped on eachsurface of the electrolyte membrane of Example 1 before being subjectedto the hydrophilization treatment. The excess dispersion was struck offthe surface of the electrolyte membrane using a polyethyleneterephthalate film. An MEA was prepared in otherwise the same manner asin Example 1. The amount of the supported electrode catalyst was 1.52mg/cm².

Performance Evaluation:

The MEAs obtained in Example 1 and Comparative Example 1 were evaluatedby observing the surface of the electrolyte membrane with a scanningelectron microscope before the gas diffusion layer was disposed. Theresults are shown in FIG. 5 (Example 1) and FIG. 6 (Comparative Example1).

The electron micrograph image of FIG. 5 reveals many fine white spots onthe periphery of ion-conductive domains, which are believed to be causedby adhesion of much catalyst. It is seen that the catalyst adheresselectively on the ion-conductive domains, avoiding thenon-ion-conductive domains. Mapping of the microscopic field by EDSascertained selective existence of platinum and sulfur on the surfacesites corresponding to the ion-conductive domains. The platinum and thesulfur originate in the catalyst and the perfluorocarbonsulfonic acidresin contained in the spray liquid, respectively. Accordingly, theresults of the mapping prove that the spray liquid adhered selectivelyto the surface sites corresponding to the ion-conductive domains.

In contrast, FIG. 6 reveals adhesion of much particulate catalyst to thenon-ion-conductive domains as well. It was confirmed as a result ofmapping that platinum and sulfur were distributed almost uniformly overthe entire microscopic field of view. The results of mapping indicatethat application of the spray liquid was not selective between theion-conductive and the non-ion-conductive domains.

Example 2

The same electrolyte membrane as prepared in Example 1 was used, exceptthat the hydrophilization treatment (ion conductivity activationtreatment) was not conducted. The electrolyte membrane was substantiallyfree from dissociated protons. Ion exchanged water was dropped using amicrosyringe on one surface of the electrolyte membrane in adiscontinuous manner in an amount of 80 μl/cm². Before the ion exchangewater evaporated, ESD was carried out on the electrolyte membrane usingthe same spray liquid and ESD conditions as in Example 1, except forchanging the spray liquid feed rate to 10 μl/min and the spray time to30 seconds. An MEA was prepared in otherwise the same manner as inExample 1. The MEA had the catalyst electrode adhered selectively to thesites where ion exchanged water had been dropped to form electrodecatalyst adhesion regions. The MEA had the same shape of adhesionregions on the opposite sites across the electrolyte membrane.

Example 3

Nafion (a registered trademark of Du Pont) 117 was used as anelectrolyte membrane. The electrolyte membrane was vacuum driedovernight to become substantially free from dissociated protons. Ionexchanged water was dropped using a microsyringe on one surface of theelectrolyte membrane in a discontinuous manner in an amount of 30μl/cm². An MEA was prepared in the same manner as in Example 2, exceptfor changing the spray liquid feed rate to 20 μl/min (the spray time was30 seconds). The resulting MEA had the catalyst electrode adheredselectively to the sites where ion exchanged water had been dropped toform electrode catalyst adhesion regions. The MEA had the same shape ofadhesion regions on the opposite sites across the electrolyte membrane.

Example 4 and Comparative Example 2

Nafion (a registered trademark of Du Pont) 117 was used as anelectrolyte membrane. A 25 mm side square was cut out of the electrolytemembrane and was made electrically conductive by boiling in an 1Nsulfuric acid aqueous solution and then in pure water each for one hourin a usual manner. A 10 mm side square of gold foil having conductingwire connected to the back side thereof was bought into intimate contactwith the central portion of one surface of the electrolyte membrane. Theelectrolyte membrane was placed on an insulating polyethyleneterephthalate film having a pinhole, with the metal foil facing the filmand with the conducting wire run through the pinhole and kept from beingexposed. The conducting wire was connected to the negative pole of thepower source shown in FIG. 3. In this state, ESD was carried out in thesame manner as in Example 3. As result, an electrode catalyst wasdeposited only on the area of the electrolyte membrane that correspondedto the gold foil (Example 4). The amount of the electrode catalystdeposited was 1 mg/cm².

For comparison, the electrode catalyst was deposited only to the area ofthe electrolyte membrane that corresponded to the gold foil by using acompressed air sprayer (Spray-Work, available from Tamiya Inc.) in placeof ESD. The amount of the electrode catalyst adhered was the same as inExample 4.

The two samples obtained were tested for peel strength of the electrodecatalyst as follows. The samples were dried at 140° C. for 30 minutes.Adhesive tape (from Sumitomo 3M Ltd.) was stuck to the electrodecatalyst adhesion region of each sample and stripped off. The area ofthe electrode catalyst transferred to the adhesive tape was observedwith the naked eye. The results are shown in FIG. 7. As is apparent fromthe results of FIG. 7, the sample of Example 4 suffered from lesspeeling of the electrode catalyst than that of Comparative Example 2.This means that the electrode catalyst applied by electrostatic spraydeposition forms a catalyst layer with high adhesion to the electrolytemembrane and hardly comes off.

Example 5 and Comparative Example 3

Nafion (a registered trademark of Du Pont) 112 was used as anelectrolyte membrane. A 4 cm side square was cut out of the electrolytemembrane. An electrode catalyst was deposited on the central portionmeasuring 2.2 cm by 2.2 cm on each surface of the electrolyte membranein the same manner as in Example 4 (Example 5). The amount of theelectrode catalyst deposited was 1 mg/cm². For comparison, the electrodecatalyst was adhered to the above-described electrolyte membrane in thesame manner as in Comparative Example 2 (Comparative Example 3). Theamount of the electrode catalyst adhered was the same as in Example 5.

Each of the resulting two samples was dried at 140° C. for 30 minutes.Carbon cloth (from Electrochem Inc.) having been made water-repellentwas overlaid as a gas diffusion layer on each surface of the sample tomake an MEA. The MEA was assembled into a single cell (PEFC SS-J fromChemix Co., Ltd.). Humidified pure oxygen and pure hydrogen were fed tothe positive electrode and the negative electrode, respectively, each ata rate of 10 ml/min, and power generation characteristics of the fuelcell at 60° C. were measured. As a result, the sample of Example 5showed an open circuit voltage of 0.93 V and a maximum power output of120 mW/cm². The sample of Comparative Example 3 had an open circuitvoltage of 0.92 V, but its maximum power output was only 80 mW/cm². Itis seen from these results that the selective deposition of theelectrode catalyst to the electrolyte membrane by electrostatic spraydeposition promises superior power generation performance.

Example 6

A polycarbonate porous film having a thickness of about 13 μm, anaverage pore size of 5 μm, and a porosity of 15% (Nuclepore fromWhatman) having its pores filled with Nafion® was prepared.Specifically, the porous membrane was immersed in a 20 wt % Nafionsolution and irradiated with ultrasonic waves to fill the pores with theNafion solution. The porous film was taken out of the Nafion solution,slowly dried in the presence of alcohol vapor, and finally dried invacuo at 140° C. for 30 minutes to obtain a pore-filling electrolytemembrane having its pores filled with Nafion. An electrode catalyst wasdeposited on one surface of the electrolyte membrane in the same manneras in Example 1. FIG. 8 presents a scanning electron micrograph taken ofa cross-section of the resulting sample. Line segment 1 indicates wherea vacant pore had been and was filled with Nafion, and line segment 2indicates the polycarbonate matrix. The sites indicated by line segments1 and 2 were analyzed for platinum in the thickness direction. Theresults are shown in FIG. 9. In FIG. 9, the distance coordinate agreeswith that in FIG. 8, and distance 0 μm indicates the position of thesurface on the right hand side in FIG. 8. As is apparent from theresults in FIG. 9, in the site of line segment 1 platinum is presentonly at the position of 0 μm distance, i.e., on the right hand sidesurface in FIG. 8. Platinum is absent in the site of line segment 2. Itis understood from these results that the electrode catalyst had beendeposited only on the desired sites.

Example 7 and Comparative Example 4

An electrode catalyst was deposited on the pore-filling electrolytemembrane prepared in Example 6 in the same manner as in Example 5 toobtain an MEA (Example 7). For comparison, an electrode catalyst wasdeposited in the same manner as in Comparative Example 3 to obtain anMEA (Comparative Example 4). The resulting membrane-electrode assemblieswere evaluated for power generation characteristics in the same manneras in Example 5. The sample of Example 7 showed an open circuit voltageof 0.92 V and a maximum power output of 60 mW/cm² whereas the sample ofComparative Example 4 had an open circuit voltage of 0.92 V and amaximum power output of 40 mW/cm². It is seen from these results thatthe selective deposition of the electrode catalyst to the electrolytemembrane by electrostatic spray deposition promises superior powergeneration performance.

INDUSTRIAL APPLICABILITY

As described, the present invention makes it possible to reduce theamount of an expensive catalyst including a noble metal required in thepreparation of a membrane-electrode assembly without impairing theperformance of the assembly, thereby making great contributions toreduction of the cost of solid polymer electrolyte fuel cells.

1. A membrane-electrode assembly comprising a solid polymer electrolytemembrane which has ion-conductive domains and non-ion-conductive domainsand an electrode catalyst, wherein the electrode catalyst is presentselectively on surface sites of the solid polymer electrolyte membranewhich corresponds to the ion-conductive domains rather than surfacesites of the electrolyte membrane which corresponds to thenon-ion-conductive domains, and the ion-conductive domains and thenon-ion-conductive domains extend in a width direction of the solidpolymer electrolyte membrane.
 2. The membrane-electrode assemblyaccording to claim 1, wherein the selective presence of the electrodecatalyst on the surface sites which corresponds to the ion-conductivedomains is obtainable by spraying a spray liquid which contains theelectrode catalyst and a solvent onto the surface of the solid polymerelectrolyte membrane by electrostatic spray deposition.
 3. Themembrane-electrode assembly according to claim 1, wherein the solidpolymer electrolyte membrane has a first surface and a second surfacewhich is opposite to the first surface, the solid polymer electrolytemembrane has a number of first electrode catalyst adhesion regionsdiscretely formed on the first surface thereof, and a number of secondelectrode catalyst adhesion regions discretely formed on the secondsurface thereof, the first electrode catalyst adhesion regions arepositioned opposite to the second electrode catalyst adhesion regions,each of the first and the second electrode catalyst adhesion regionsincludes the surface sites which corresponds to the ion-conductivedomains and the surface sites which corresponds to thenon-ion-conductive domains, and has the electrode catalyst which isselectively present on the surface sites which corresponds to theion-conductive domains rather than the surface sites which correspondsto the non-ion-conductive domains.
 4. The membrane-electrode assemblyaccording to claim 3, wherein the solid polymer electrolyte membrane anda pair of the electrode catalyst adhesion regions opposite each otheracross the electrolyte membrane constitute a single cell, and the singlecells are connected in series via an interconnector.
 5. Themembrane-electrode assembly according to claim 1, wherein the solidpolymer electrolyte membrane comprises a perfluorocarbonsulfonic acidresin, an electric field-oriented solid polymer ion conductor obtainableby orienting a polymer having an ionically dissociable group in anelectric field, or a pore-filling polymer obtainable by filling pores ofnon-ion-conductive porous polymer with an ion-conductive polymer.
 6. Aprocess of producing the membrane-electrode assembly according to claim1, comprising applying a spray liquid containing the electrode catalystand a solvent onto a surface of the solid polymer electrolyte membraneby electrostatic spray deposition.
 7. The process according to claim 6,wherein, under the condition in which the solid polymer electrolytemembrane is placed on an electron conductive plate, and a nozzleejecting the spray liquid is arranged to face the solid polymerelectrolyte membrane, the spray liquid is sprayed from the nozzle ontothe solid polymer electrolyte membrane with an electric field appliedbetween the electron conductive plate and the nozzle.
 8. The processaccording to claim 7, wherein the solid polymer membrane is subjected toa hydrophilization treatment before being sprayed with the spray liquid.9. A process of producing a membrane-electrode assembly comprising thesteps of: discretely applying an ion-conductive liquid to a surface of asolid polymer electrolyte membrane which is substantially free from adissociated proton, and then applying a spray liquid containing anelectrode catalyst and a solvent onto the surface of the solid polymerelectrolyte membrane by electrostatic spray deposition to adhere theelectrode catalyst selectively to the part of the solid polymerelectrolyte membrane where the ion-conductive liquid has been applied,wherein ion-conductive domains and non-ion-conductive domains extend ina width direction of the solid polymer electrolyte membrane.
 10. Theprocess according to claim 9, wherein the ion-conductive liquid iswater, a dilute acid aqueous solution, or a lower alcohol.
 11. A solidpolymer electrolyte fuel cell comprising the membrane-electrode assemblyaccording to claim 1, and a separator which is arranged on each side ofthe membrane-electrode assembly.
 12. The membrane-electrode assemblyaccording to claim 1, wherein the catalyst contains metal particleshaving a particle size of 10 to 300 μm.
 13. The membrane-electrodeassembly according to claim 1, wherein the catalyst has a coverage of0.001 to 10 mg/cm².
 14. The process according to claim 9, wherein thecatalyst contains metal particles having a particle size of 10 to 300μm.
 15. The process according to claim 9, wherein the catalyst isapplied to have a coverage of 0.001 to 10 mg/cm².
 16. The processaccording to claim 6, wherein the electrostatic spray deposition isperformed at a voltage of 5 to 30 kV.
 17. The process according to claim9, wherein the electrostatic spray deposition is performed at a voltageof 5 to 30 kV.